Interference signal suppressor for a radio receiver

Abstract

An FM interference suppressor comprises two FM demodulators, one of which is fed with an input signal via an envelope detector and is arranged to feed a zero frequency detector and a phase inverter to produce a baseband composite signal comprising wanted and unwanted modulation. The other FM demodulator is fed directly with an input signal directly to produce a baseband signal comprising the unwanted modulation alone. A combiner is provided for subtractively combining the baseband signal and the baseband composite signal to produce the wanted modulation signal.

I claim: 
     
       1. An interference suppressor and demodulation unit for a radio receiver for suppressing an unwanted FM signal having a signal strength greater than the signal strength of a wanted FM signal, each FM signal having a respective frequency, the suppressor comprising means for providing an amplitude modulated beat signal from the wanted and unwanted FM signals, said means being coupled to a means for providing a difference frequency signal in dependence upon the amplitude modulated beat signal, a first demodulator arranged to receive the difference frequency signal and apply this through a zero frequency detector and phase reverser to provide a composite baseband signal comprising a modulation of the wanted and unwanted FM signals, a further demodulator arranged to receive the amplitude modulated beat signal, for providing a baseband signal corresponding to the modulation of the unwanted FM signal, and a combiner coupled to the first demodulator and said further demodulator, the combiner being arranged to receive the composite baseband signal and the baseband signal for providing an output signal comprising the modulation of the wanted FM signal and in which the unwanted signal has been suppressed. 
     
     
       2. An interference suppressor according to claim 1 wherein the means for providing the amplitude modulated beat signal comprises a bandpass filter. 
     
     
       3. An interference suppressor according to claim 1 wherein the difference frequency signal has an instantaneous frequency and the first demodulator comprises an instantaneous direct conversion frequency demodulator arranged to provide an output signal having a voltage which is proportional to the instantaneous frequency of the difference frequency signal. 
     
     
       4. An interference suppressor according to claim 3, wherein the instantaneous direct conversion frequency demodulator comprises a differentiator circuit arranged to receive the amplitude modulated beat signal coupled to a Hilbert transformer, the Hilbert transformer being coupled to two mixers, a clock pulse source coupled to the two mixers thereby enabling the mixers to operate in phase quadrature and a combiner coupled to the mixers for combining the output signals from the two mixers and an envelope detector coupled to the combiner. 
     
     
       5. An interference suppressor according to claim 3 comprising a zero frequency detector and phase reverser coupled to the first demodulator for reversing the phase of the output signal from the first demodulator when the respective frequencies of the wanted and unwanted FM signals are closer than a FM deviation in the wanted and unwanted signals. 
     
     
       6. An interference suppressor according to claim 5 wherein the zero frequency detector and phase reverser comprises a first switched gate circuit having a first input for receiving the output signal from the first demodulator and a second input for receiving the output signal from the first demodulator via a threshold detector, and a second switched gate circuit having a first input for receiving the output signal from the first demodulator via a phase reversing amplifier and a second input for receiving the output signal from the first demodulator via the threshold detector and a further phase reversing amplifier. 
     
     
       7. An interference suppressor according to claim 1 wherein the further demodulator comprises a limiter discriminator. 
     
     
       8. A interference suppressor according to claim 1, wherein the means for providing a difference frequency signal comprises an envelope detector. 
     
     
       9. An interference suppressor according to claim 8 wherein the envelope detector for providing the difference frequency signal for the first demodulator comprises a single port mixer. 
     
     
       10. An interference suppressor and demodulator unit for a radio receiver, the unit having means for suppressing an unwanted FM signal having a signal strength greater than the signal strength of a wanted FM signal, the unit comprising a bandpass filter for providing an amplitude modulated beat signal from the wanted and unwanted FM signals, an envelope detector coupled to said filter and arranged to produce a difference frequency signal dependent upon said beat signal, a first demodulator arranged to provide an output signal containing an addition of the modulation waveforms of the wanted and unwanted signals, a zero frequency detector and phase reverser providing modulation waveforms of the wanted and unwanted signals, and a combiner having a first input terminal connected to receive said modulation waveforms, the combiner additionally having a second input terminal which is connected to a second demodulator to which the amplitude modulated beat signal is applied, the combiner providing a signal at an output terminal which comprises the modulation part of the wanted FM signal and in which the unwanted signal has been suppressed. 
     
     
       11. An interference suppressor unit according to claim 10, in which said first demodulator is an instantaneous direct conversion frequency demodulator arranged to provide an output signal having a voltage which is proportional to the instantaneous frequency of the amplitude modulated beat signal. 
     
     
       12. An interference suppressor unit according to claim 11, in which the frequency demodulator comprises a differentiator circuit coupled to a Hilbert transformer, said transformer having output leads which are coupled to a phasing type single side band generator, said generator comprising a pair of mixers which are arranged for operation in phase quadrature by means of a phase shifter and signals from a RF clock pulse source, and the mixers having output leads which are coupled to a combiner, in which an output lead from said combiner is connected to an envelope detector, an output from said envelope detector then serving as an output terminal of the resulting frequency demodulator circuit. 
     
     
       13. An interference suppressor and demodulator unit for a radio receiver, the unit having means for suppressing an unwanted FM signal having a signal strength greater than the signal strength of a wanted FM signal, the unit comprising a bandpass filter for providing an amplitude modulated beat signal from the wanted and unwanted FM signals, an envelope detector coupled to said filter and arranged to produce a difference frequency signal dependent upon said beat signal, a first demodulator comprising an instantaneous direct conversion frequency demodulator arranged to provide an output signal having a voltage which is proportional to the instantaneous frequency of the amplitude modulated beat signal, a zero frequency detector and phase reverser comprising a first switched gate circuit having a first input for receiving the output signal from the first demodulator and a second input for receiving the output signal from the first demodulator via a threshold detector, and a second switched gate circuit having a first input for receiving the output signal from the first demodulator via a phase reversing amplifier and a second input for receiving the output signal from the first demodulator via the threshold detector and a further phase reversing amplifier thus providing modulation waveforms of the wanted and unwanted signals, and a combiner having a first input terminal connected to receive said modulation waveforms, the combiner additionally having a second input terminal which is connected to a second demodulator to which the amplitude modulated beat signal is applied, the combiner providing a signal at an output terminal which comprises the modulation part of the wanted FM signal and in which the unwanted signal has been suppressed. 
     
     
       14. An interference suppressor and demodulator unit for a radio receiver, the unit having means for suppressing an unwanted FM signal having a signal strength greater than the signal strength of a wanted FM signal, the unit comprising a bandpass filter for providing an amplitude modulated beat signal from the wanted and unwanted FM signals, an envelope detector coupled to said filter and arranged to produce a difference frequency signal dependent upon said beat signal, a first demodulator comprising an instantaneous direct conversion frequency demodulator which comprises a differentiator circuit coupled to a Hilbert transformer, said transformer having output leads which are coupled to a phasing type single sideband generator, said generator comprising a pair of mixers which are arranged for operation in phase quadrature by means of a phase shifter and signals from a RF clock pulse source, and the said mixers having output leads which are coupled to a combiner, in which an output lead from said combiner is connected to an envelope detector, an output from said envelope detector then serving as an output terminal of the resulting frequency demodulator circuit, a zero frequency detector and phase reverser comprising a first switched gate circuit having a first input for receiving the output signal from the first demodulator and a second input for receiving the output signal from the first demodulator via a threshold detector, and a second switched gate circuit having a first input for receiving the output signal from the first demodulator via a phase reversing amplifier and a second input for receiving the output signal from the first demodulator via the threshold detector and a further phase reversing amplifier thus providing modulation waveforms of the wanted and unwanted signals, and a combiner having a first input terminal connected to receive said modulation waveforms, the combiner additionally having a second input terminal which is connected to a second demodulator to which the said amplitude modulated beat signal is applied, combiner providing a signal at an output terminal which comprises the modulation part of the wanted FM signal and in which the unwanted signal has been suppressed.